1. Field of the Invention
The present invention relates to the field of radio transmissions and in particular it relates to the space diversity systems. Still more in particular, it concerns a method and an apparatus for compensating, in an automatic and digital manner, the relative delay between the two (or more) signals received simultaneously.
2. Description of the Prior Art
In a high-capacity radio transmission system the channel distorting effects are well known, mainly due to the multipath phenomenon: the receiving antenna can in fact receive, along with the wished signal, a delayed replica thereof, caused by the reflection of the transmitted signal from tropospheric layers having unusual physical characteristics or by the reflection from orographic obstacles. Such a corruptive effect is known in the leterature as selective fading. Under exceptionally unfavourable conditions the fading can even result in the radio system to be out-of-order, since the received signal becomes no longer intelligible.
A first, possible and widely used in practice, countermeasure is represented by the adaption of an adaptive equalizer inside the demodulation apparatus. This solution, sometimes, may be not enough in the case of radio spans extraordinarily long or installed under exceptionally unfavourable geographic conditions.
Another possible alternative consists in providing a space diversity system, based upon the transmission of a signal which is received simultaneously by two or more different antennas (hereafter, two antennas will be considered by way of a non limiting example). The operating philosophy of the space diversity system consists exactly in sending the same information to the receiver by means of two distinct signals (one will be called “main” and the other will be called “diversity”). The effectiveness of this method depends on the fact that if the antennas are sufficiently spaced out in height, the received signals can be deemed uncorrelated and hence it is extremely unlikely that both signals exhibit the same quality at the same instant.
Two main methods for processing the pairs of received signals are known: switching and combination. Switching is based on the selection, theoretically at every instant, of the best of the two signals through a suitable criterion (typically the BER or Bit Error Rate).
An approach deemed more effective consists in processing the two diversity signals by properly combining them. The procedure often used in this case is the one illustrated in FIG. 1, in which the two signals, main and diversity, suitably sampled, are the inputs of two FSEs (Fractionally Spaced Equalizers) whose output is summed and represents the result of the combination.
However, because of the different positioning height of the two antennas at the receiving tower, of the different length of the waveguides or in any case of the various connection cables, the main and diversity signals may reach the samplers of FIG. 1 delayed one to each other. In order to realize the combination in an effective manner it is necessary to compensate for such a delay. Generally, once the delay has been measured by means of proper instruments, it is compensated during the installation of the radio link by adding to one of the two signals a cable length such that the transit time is equal to the delay to be compensated of by means of proper analog delay cells suitably adjustable.
Unfortunately, this solution has the drawback of entailing a high cost and of the need to carry out the calibration on the field (thus requiring long times to reach the antennas).